Ignore {knitr} chunks that use `Rcpp` engine?

[IndrajeetPatil]

In a way, a follow-up to #312.

Should {styler} ignore {knitr} R code chunk with Rcpp engine?

For example, if I try to style an .Rmd file containing the following chunk,

```{r, engine='Rcpp'}
#include <Rcpp.h>
using namespace Rcpp;
```

it will (understandably) produce the following error:

Error in `parse_safely()`:
! <text>:2:7: unexpected symbol
1: #include <Rcpp.h>
2: using namespace
         ^

But this file might also contain a number of other R code chunks with engine = 'R', and none of them will be styled due to the offending Rcpp engine chunk.

[lorenzwalthert]

Thanks. The quick fix I beliebe is just not to use r at the beginning, as the first unnamed argument of a code chunk header seems to get matched with the engine. In particular for Rcpp:

```{Rcpp}
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector timesTwo(NumericVector x) {
  return x * 2;
}

However, to solve this more generically, we should probably rely on {knitr} to parse the code chunk header. This would also allow for #839, since we'd have keys and values we can easily format. Since we already import {knitr} in Suggests and everyone working with R Markdown documents already has it, I would be happy to depend on it for this functionality. Not sure {knitr} exports the functionality for parsing the headers, probably not. But maybe we can convince them of that use case. First step would anyways be to find the relevant code in {knitr} that parses the headers. Are you interested in contributing this?

[lorenzwalthert]

Also note other issues that have been filed with similar requests #832, #312.

[IndrajeetPatil]

First step would anyways be to find the relevant code in {knitr} that parses the headers. Are you interested in contributing this?

Sure, I can try to find the relevant code.

[IndrajeetPatil]

Btw, every time I comment on styler issue, I get this GitHub notification and I have no idea what it's about:

[IndrajeetPatil]

The parameters from chunk header are parsed using an internal function called parse_params():

knitr:::parse_params("a=1, engine='Rcpp'")
#> $a
#> [1] 1
#> 
#> $engine
#> [1] "Rcpp"
#> 
#> $label
#> [1] "unnamed-chunk-3"

knitr:::parse_params("Rcpp, foo, a=1,")
#> $label
#> [1] "Rcpp, foo"
#> 
#> $a
#> [1] 1

^{Created on 2022-03-10 by the reprex package (v2.0.1.9000)}

[lorenzwalthert]

Thanks, I'll file an issue in {knitr} and ask about exporting this function. Really strange these emails you get...

[IndrajeetPatil]

Really strange these emails you get...

This traces to following GHA, which is triggered on issue comment:

styler/.github/workflows/pr-commands.yaml

Lines 1 to 3 in b70aebe

	on:
	issue_comment:
	types: [created]

[lorenzwalthert]

strange. Because I never get any e-mail...

[lorenzwalthert]

Here's all we need to know going forward: yihui/knitr#2112